Brandwacht River stabilisation project

from Water & Sanitation Africa May/June 2022

Lesotho Highlands Water Project – Phase II: An update

Alien vegetation, riverbank erosion and large quantities of sediment washing downstream prompted the Brandwacht River stabilisation project.

By Hans King, owner of Hans King SRS

Afew kilometres from Hartenbos, near George in the Eden District of the Western Cape, lies the Brandwacht River. It flows southwards from the Outeniqua Mountains, and discharges into the Atlantic Sea via the Klein Brak River.

The problem Over the past 15 years, there has been a marked increase in the presence of black wattle as well as other species of alien vegetation such as the Spanish reed. The riverbank erosion at sites opposite where the black wattle is growing has increased in severity to the point where the right bank has moved completely to where the left bank was 10 years ago.

Many small farmers along the river produce a variety of crops. Not only was agricultural soil being lost on the bank of the river, but large quantities of sediment were washing downstream as a result of the impact of the alien vegetation. On-site inspections and historical satellite imagery revealed that the river became ever more degraded as one progressed down its course, largely due to bulldozing and the flourishing of invasive alien vegetation.

Due to limited funds, a 3 km stretch was selected for rehabilitation where there was intense alien vegetation growth and serious soil loss and sediment load mobilisation were taking place. It was believed that by reducing the sediment flow down the river from the target area, the whole riparian community downstream would benefit from the work.

Objectives The primary objective of the rehabilitation work was to prevent the abnormal movement of sediment down the river (in other words sediment that was mobilised by the ingress of invasive alien vegetation), and therefore protect the agricultural production potential of land downstream of the site. Rehabilitation work would also: • protect agricultural production potential on the site itself (by preventing the washing away of the riverbanks) • provide work opportunities for unemployed locals during the project, and for the maintenance of the river after the project • protect the natural environment by the removal of alien vegetation and the partial restoration of the natural flora at the worksite • protect local road and bridge infrastructure, which impacts on the local agricultural economy.

Interventions A detailed topographical survey of the 3 km long stretch of river was carried out, and a hydrological study of the catchment was done. This information was used to determine flood levels, to design a rehabilitated river channel (along the route that existed about

Invasive alien vegetation pushing the oncoming river into the road fill

A typical groyne structure

10 years previously), and to design groyne structures to support the rehabilitated channel.

The width and shape of the rehabilitated channel were designed so that flow velocities during floods were mild enough not to promote erosion, especially once the indigenous wetland vegetation had been re-established.

A total of 27 groynes were chosen as river training structures (as opposed to riprap or other longitudinal protection), partly because it was a lot more economical, but also because the use of groynes promoted a wider and slowerflowing watercourse during floods. Furthermore, the spaces between the groynes were used for trapping sediment and the establishment of indigenous wetland vegetation.

The height of the groyne structures above the riverbed was kept low so that large sections of the groynes would be overtopped during moderate floods and double the river’s flow width.

Approximately 9 000 m³ of rock was used to fill gabion baskets. Rock-filled gabion baskets were selected as the construction material for two reasons: 1. The material was flexible enough to accommodate foundation

movements should this occur during an extreme flood. 2. Rock-filled gabions are easy to work with, and this facilitated the employment of a labour force of at least 70% local persons.

Design and construction challenges The design technique for groynes developed in the Western Cape has often been used for cobble-bed rivers; however, the Brandwacht River has sand-beds.

It was expected that the sediment of a sand-bed river would be mobilised to a greater depth during a flood than a cobble-bed river. To make sure that the tips of the groynes at Brandwacht would be stable during flooding, it was therefore essential to keep the foundations at least 1 m below the lowest part of the pools in the surveyed pool-and-riffle sequence. This caused problems during construction, as the anchor block under the mattress around the foundation of the groyne was at least 2 m below the riverbed.

The contractor found that conventional techniques of pumping water out of the excavation so that the anchor block could be installed did not work, as the inflow of subsoil water was too strong. It was then decided to employ a curtain of well points around the excavation and a very large pump, which dropped the water table enough to enable the construction to go ahead as planned.

Post the completion of construction, several small floods have been experienced, but nothing significant to test the project yet. The indigenous wetland vegetation has re-established. The banks of the river are now stable.